1. Field of the Invention
This invention relates to a chemical substance-sensing element forming a chemical substance sensor capable of detection and quantitative analysis of a chemical substance as well as a chemical substance-sensing device.
2. Prior Art
Chemical sensors are known which selectively adsorb or react with a particular chemical substance existing in an ambient environment and detects the type and concentration of the chemical substance from a change of electrical signal due to a change of resistivity or dielectric constant through such adsorption or reaction.
These chemical sensors are constructed from such materials as semiconductors, ceramics, and polymers and are in fact used for the detection and quantitative analysis of water vapor, various gases, various ions or the like.
Sensors using cholesteric liquid crystal are also known, see Japanese Patent Application Kokai Nos. 76094/1978 and 42685/1983, Tsukahara, Mechatronics, 6, 3 (1981), 103, Sensor Gijutsu (Sensor Technology), 7, 7 (1987), 62; ibid., 7, 11 (1987), 97; and ibid., 7, 12 (1987), 87.
These sensors utilize a color change of liquid crystal.
With the recent advance of office automation equipment, there is a need for sensors such as humidity sensors which can perform stably and precisely even in an environment having an increased electric field as found in copying machines. Sensors utilizing optical detection and quantitative analysis are advantageous for such purposes.
For such sensors, there is known an optical detection and quantitative analysis method which utilizes a change of optical absorption or fluorescence quantity (or extinction of fluorescence) of a certain dye or compound reacting with a gaseous substance or forming a charge transfer complex.
One known method utilizes a color change due to reaction of N,N-dimethylaniline with O.sub.2 (see Japanese Patent Application Kokai No. 100337/1982).
Another known method utilizes a color change due to reaction of poly-2-para-(methacryloylaminophenyl)-5-phenyl-1,3-oxazole with NH.sub.3 and amines (see Japanese Patent Application Kokai No. 202334/1985).
Also known is a method utilizing a fluorescence change due to reaction of tris(4,7-diphenyl-1,10-phenanthroline) Ru complex with 0.sub.2 (see Japanese Patent Application Kokai No. 178646/1986).
Additional known methods utilize an absorption change of a Thymol Blue film by NH.sub.3 (see Nikkei Sangyo Shinbun, Dec. 14, 1987) and a fluorescence change of an LB film of a squalirium dye and a fatty acid by NOx (Japan Chemical Society, Spring Meeting Preprint, 3IIH07, 1988 and Nikkei Sangyo Shinbun, Jun. 10, 1988).
It has also been contemplated to apply metal or metal salt films in order to utilize optical reflection though such reports are a few.
For example, it is proposed to utilize reaction of palladium potassium sulfite with CO (see Japanese Patent Application Kokai No. 79141/1981), reaction of HgBr.sub.2 with AsH.sub.3 (see Japanese Patent Application Kokai No. 42645/1985), and corrosion of a metal film by SF.sub.4 (see Japanese Patent Application Kokai No. 8066/1987).
Known humidity sensors utilize a change in refractive index of an optical fiber clad with a hygroscopic material (see Japanese Patent Application Kokai Nos. 217744/1986, 9255/1987, and 204143/1987);
a change in color of an inorganic material upon absorption of humidity (see Japanese Patent Application Kokai Nos. 80190/1979 and 139478/1981);
a change in coefficient of absorption or a change in scattering intensity or color development of a hygroscopic resin or the like (see Japanese Patent Application Kokai Nos. 67738/1981, 110039/1981, and 216936/1983);
a change of irregular reflection using a transparent member having an irregular surface and a hygroscopic agent (see Japanese Patent Application Kokai No. 204742/1984); and
reflected light and transmitted light associated with a mirror consisting of two dielectric layers having different refractive indexes (see Japanese Patent Application Kokai No. 39744/1987).
Nevertheless, no proposal has been made as to the utilization of reflection of a dye film.
The above-mentioned sensors constructed from semiconductors, ceramics, and polymers have a drawback that the response is slow because of diffusion of a gas to be detected through a thick film. Many of them require regeneration. Often a change of resistance is detected by electric means which experiences a large change with time. In most cases, manufacture of such elements requires complicated steps and the detection circuit is complicated.
The sensors using cholesteric liquid crystal utilize a color change in proportion to a pitch change and have drawbacks including a widely varying or inconsistent wavelength distribution of light reflection, a low sensitivity, and considerable temperature dependency.
Utilization of a change of optical absorption or fluorescence quantity of a dye or the like has a drawback that the dye or the like must be able to selectively bond or react with a substance to be detected and quantitatively analyzed such as a gaseous substance. A highly advanced technique or a complicated step is necessary in manufacturing such an element. For example, in an element utilizing a change in fluorescence of an LB film, dye molecules must be oriented to high accuracy in applying the LB film. In an element utilizing a change in absorption of a Bromothymol Blue film, a reflective film must additionally be formed.
Further, sensors utilizing a metal or metal salt film have drawbacks that the reaction is irreversible in all cases, and the combination of a sensing material and a substance to be detected such as a gaseous substance is limited.